There are occasions in the oil and gas industry when a gas may be pumped downhole together with a liquid phase such as a treatment fluid or a drilling fluid. In particular it may be useful to pump nitrogen gas downhole during drilling or during well workover operations. There could be a variety of purposes for pumping downhole a gas with a liquid phase. Such purposes might include helping to lift liquids back to the surface and/or lowering the pressure exerted by the combination of fluids against fragile wellbores. "Underbalanced" drilling, for instance, typically utilizes a gas added to a drilling fluid to "underbalance" the pressure between the drilling fluid and portions of the formation that are open downhole.
One illustration of a well workover application where it might be useful to pump gas downhole is in rotary jet cleaning. In rotary jet cleaning a liquid is pumped downhole and out of a rotary jet cleaning tool. Gas could be advantageously added to the liquid in so far as the gas could help lift and circulate the cleaning liquid back up hole, possibly enhancing the liquid's capacity to carry debris. Drilling with a downhole motor and rotary jet drilling might have similar applications in which it could be advantageous to add gas to a working liquid, at least for lifting purposes. However, running mixed gas/liquid phase through a downhole hydraulically powered motor or other apparatus, such as a downhole drilling motor or a rotary jet cleaning tool, is not favored. The gas/liquid phase neither optimizes downhole motor performance nor optimizes maintenance of the motor parts. Sending a mixed gas/liquid phase through a rotary jet cleaner, in addition, may result in the loss of optimum cleaning power.
One aspect of the instant invention, therefore, is a methodology and apparatus affording the ability to remove a gas phase at or in a bottomhole assembly (BHA) when the presence of the gas downhole could be helpful but when it would also be useful to prevent the gas from invading and damaging elastomers in a drilling motor and/or to optimize the cleaning performance of a rotary jet cleaner by excluding a gas phase.
Existing commercially available downhole liquid/gas flow separators seem to be designed for separating production fluids. These are fluids flowing up either under natural pressure or being pumped. These separators appear optimized for narrow ranges of gas volume fraction and/or for high values of entry or initial gas volume fraction. They appear typically optimized for entry gas volume fractions of between 90% and 100% and for exit gas volume fractions of between 15% and 50%. See U.S. Pat. No. 5,482,117, column 1, line 58. These entry ranges are too high and too narrow to be useful for generally separating fluid mixtures, in particular gas/liquid mixed phase fluids, that might be pumped downhole in either a drilling application or in a jetting application or in other workover applications. The exit volume fractions are also too high.
The problems involved in cost effectively, efficiently and sufficiently separating pumped fluids flowing downhole are different from the problems involved in sufficiently separating well fluids produced into a well to be flowed or pumped up.
A further aspect of the present invention includes the design of an efficient and effective downhole fluid phase separator, which includes gas/liquid separating, that can effectively and efficiently operate without excessive loss of pressure to the fluid pumped downhole and can operate over a range of supplied gas volume fractions that might run from 10% through 90%. Further, the separator must not be too long. Important aspects of the invention include the length of the separator, ideally below three (3) feet, and the pressure drop caused by the tool, preferably below 10% of the supplied fluid pressure. The outside diameter of the tool will be limited by the diameter of the wellbores through which the bottomhole assembly is designed to run. Simplicity of operation and the absence of moving parts are further advantageous features found in embodiments of the instant design which enhance the value of the tool.
Disclosed herein is a preferred embodiment for a fluid (particularly including liquid/gas) phase separator for use on fluid mixtures pumped downhole, and its methods of use. One prime application lies with coiled-tubing-based downhole operations. The device separates fluids by density, including nitrified treatment fluids and nitrified drilling fluids. The fluids are separated into at least two constituent phases or portions. The device can be structured and designed to optimize the separation of one stream, such as a liquid stream, so that that stream is relatively free of a second fluid, such as a gas. "Relatively" in the instant environment means at least 75% free. Preferred embodiments have achieved significantly greater percentages of separation.
For purposes herein fluids are distinguished or characterized as separate fluids by their density, or at least by their capacity to be separated by density. Use of the term fluid mixture implies a mixture of fluids with different densities or at least a mixture of fluids that can be separated into at least two streams by density. The disclosed tool and method separate a fluid mixture into at least two fluid streams by density and subsequently permit directing each stream to a different path in accordance with useful applications.
In the present invention separating fluids by density is preferably achieved by inducing centrifugal acceleration, or a swirling flow path, to a moving fluid stream. Preferably a significant annular flow is first or also induced within the limits of space available. Preferably also a gradually expanding flow path in terms of cross-sectional area of flow is defined in a chamber that receives centrifugally accelerated fluids.
It should be understood that the distinct stages of the disclosed preferred embodiment herein could be overlapped in alternate designs. Distinct steps disclosed by the preferred embodiment could be made simultaneous or partially simultaneous. The instant design facilitated testing of functionality. With the present design the length of the tool has been shown to be able to be satisfactorily minimized, as has the loss of head pressure for the pumped fluids due to the separation process. High efficiencies in the separation of gas from liquid have been shown to be achievable.
In regard to gas/liquid separation, which is a prime application, shop tests have indicated that a separation efficiency can be achieved such that less than three percent (3%) of the original gas is left in a liquid fluid stream. This was achieved with a tool having less than three feet of length. (More than 3% of the original liquid may or may not be left in the gas, as this may not be a critical parameter.) It will be understood that multiple stages could be utilized to improve further gas separation efficiency. Alternately, gas separation efficiency could be improved by accepting more liquid in the gas discharge stream.
The combination of features designed into preferred embodiments of the tool, and designed into preferred embodiments of the methodology disclosed, advantageously provides the ability to function effectively, efficiently and economically under significant size and performance limitations, as required for downhole operations.